1. Field of the Invention
An object of the present invention is a method for the fabrication of an impregnated cathode, and a cathode obtained by this method. It can be applied to the making of cathodes for electron tubes, microwave tubes, television camera tubes, display tubes etc.
2. Description of the Prior Art
An impregnated cathode consists of a porous body made of a refractory metal such as tungsten, impregnated with a mixture of alkaline earth metals or their oxides, capable of giving, on the surface of the cathode, free alkaline earth atoms.
The refractory metal body of the impregnated cathode may be made by compressing a finely divided powder of the metal by means of an isostatic press or a uniaxial press. The compact bodies obtained are then heated under hydrogen, at high temperature, in order to sinter the particles together and increase the density of the porous body.
To make it easier to machine the porous body, it is infiltrated with copper or plastic and then machined to the desired shape. Subsequently, the copper or plastic is removed by heating or by being dissolved in an acid.
In order to shorten the manufacturing process, the cathode can be also pressed into shape at the very first stage, taking into account size variation, that may occur during sintering.
The electron emissivity of these cathodes can be further increased if the porous bodies are made either with a mixture of tungsten with osmium or with other elements of platinum ore, or, again, with a mixture of tungsten and scandium oxide (Sc.sub.2 O.sub.3) or oxides of other rare earths.
The porous body thus obtained is brazed on a molybdenum sleeve used to hold, on one side, the emissive pellet and, on the other side, the heater which is potted in alumina and enables the cathode to be heated.
The pores of the porous body can then be filled with barium and calcium aluminates. For this operation, the porous body is held in close contact with an aluminate composition which is brought under a reducing atmosphere, to a temperature which is greater than its melting point. The contact is provided either by submerging the porous body in aluminate or by placing the aluminate on the porous body. Upon melting, the aluminate diffuses by capillary action or flows into the open pores, and fills them completely. After cooling, the porous body is generally filled, to 90 or 100%, by the solidified aluminate. Afterwards the cathode is cleaned mechanically and chemically, in order to remove the aluminate residues which remain stuck to the surfaces.
The cathode is then mounted in a system under vacuum. It is then activated at a temperature at which the barium and calcium aluminates release barium oxide. Metallic barium is produced, in the zones where the aluminate is in contact with the refractory metal, by reduction of barium oxide vapor on the surface of the tungsten. The metallic barium reaches the end of the pores and diffuses throughout the emitting surface where it forms, with oxygen, a surface monolayer which favors electron emissivity by lowering the work function of the substrate.
Moreover, the deposition, on the emissive surface of these impregnated cathodes, of a thin film of a few thousand angstroms thick, of osmium, iridium, ruthenium or an alloy of these bodies with tungsten, may improve emissivity by a factor of about 3.
The lifetime of an impregnated cathode of this type depends on the supply of barium to the surface from within the pores, throughout the working life of the cathode. This process of supplying barium is, however, countered by the residues of reactions which form gradually and clog the pores, restricting the formation of free barium and its migration to the surface. At the end of the lifetime of these cathodes, it is observed that only a part of the barium reserve contained in the porous body has been used.
This drawback is related to standard impregnation procedures which result in compact filling, by the aluminates, of the open pores in their entirety, from the bottom of the porous body to its surface. Furthermore, the exact composition of the impregnating agent which occupies the pores is not known with precision, and depends very strongly on the conditions of impregnation.
The present invention is aimed precisely at overcoming these drawbacks. To this end, it proposes an original impregnation process. According to the invention, barium aluminate, calcium aluminate and, possibly, scandium aluminate are infiltrated in the sol state, namely in the state of stable colloidal suspension in an aqueous medium or in an organic solvent; and, after being applied by a sol-gel procedure, the compound then lines or loosely occupies the pores of the porous body throughout its thickness.
This distribution of the impregnating element increases the barium reserve which may effectively participate in the coating of the emissive surface. The cathodes obtained by the method of the invention thus have their lifetimes increased.
The compound which occupies the pores is of a known composition and is uniform at the atomic level. It can be mentioned that the application of the sol-gel procedure is already known in the fabrication of impregnated cathodes, but under totally different conditions of application. In fact, it has been proposed to prepare the barium aluminate and calcium aluminate powder by the sol-gel procedure instead of by the standard calcination of a mixture of carbonate and alumina. The sol-gel procedure has the advantage of giving very fine grain size and very homogeneous composition. The powder is brought to a temperature which is higher than its melting point. The impregnation of the porous body with tungsten then occurs according to the standard technique, described further above, by immersion in the melted mixture. The obtaining of the powders by the sol-gel method is described, for example, in the article by J. C. Bernier, "Sol-gel Processing for the Synthesis of Powders for Dielectrics" in "Powder Metallurgy International", Vol. 18, No. 3, 1986, pp. 164-168.